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Date:   Thu, 30 May 2019 16:41:11 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] perf record: collect user registers set jointly with
 dwarf stacks

Em Thu, May 30, 2019 at 10:03:36PM +0300, Alexey Budankov escreveu:
> 
> When dwarf stacks are collected jointly with user specified register
> set using --user-regs option like below the full register context is
> captured on a sample:
> 
>   $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP -- stack_test2.g.O3
> 
>   188143843893585 0x6b48 [0x4f8]: PERF_RECORD_SAMPLE(IP, 0x4002): 23828/23828: 0x401236 period: 1363819 addr: 0x7ffedbdd51ac
>   ... FP chain: nr:0
>   ... user regs: mask 0xff0fff ABI 64-bit
>   .... AX    0x53b
>   .... BX    0x7ffedbdd3cc0
>   .... CX    0xffffffff
>   .... DX    0x33d3a
>   .... SI    0x7f09b74c38d0
>   .... DI    0x0
>   .... BP    0x401260
>   .... SP    0x7ffedbdd3cc0
>   .... IP    0x401236
>   .... FLAGS 0x20a
>   .... CS    0x33
>   .... SS    0x2b
>   .... R8    0x7f09b74c3800
>   .... R9    0x7f09b74c2da0
>   .... R10   0xfffffffffffff3ce
>   .... R11   0x246
>   .... R12   0x401070
>   .... R13   0x7ffedbdd5db0
>   .... R14   0x0
>   .... R15   0x0
>   ... ustack: size 1024, offset 0xe0
>    . data_src: 0x5080021
>    ... thread: stack_test2.g.O:23828
>    ...... dso: /root/abudanko/stacks/stack_test2.g.O3
> 
> After applying the change suggested in the patch the sample data contain
> only user specified register values. IP and SP registers (DWARF_MINIMAL_REGS)
> are collected anyways regardless of the --user-regs value provided from
> the command line:

Applied, changed the subject and description to:

perf record: Allow mixing --user-regs with --call-graph=dwarf

When DWARF stacks were requested and at the same time that the user
specifies a register set using the --user-regs option the full register
context was being captured on samples:

  $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP -- stack_test2.g.O3

  188143843893585 0x6b48 [0x4f8]: PERF_RECORD_SAMPLE(IP, 0x4002): 23828/23828: 0x401236 period: 1363819 addr: 0x7ffedbdd51ac
  ... FP chain: nr:0
  ... user regs: mask 0xff0fff ABI 64-bit
  .... AX    0x53b
  .... BX    0x7ffedbdd3cc0
  .... CX    0xffffffff
  .... DX    0x33d3a
  .... SI    0x7f09b74c38d0
  .... DI    0x0
  .... BP    0x401260
  .... SP    0x7ffedbdd3cc0
  .... IP    0x401236
  .... FLAGS 0x20a
  .... CS    0x33
  .... SS    0x2b
  .... R8    0x7f09b74c3800
  .... R9    0x7f09b74c2da0
  .... R10   0xfffffffffffff3ce
  .... R11   0x246
  .... R12   0x401070
  .... R13   0x7ffedbdd5db0
  .... R14   0x0
  .... R15   0x0
  ... ustack: size 1024, offset 0xe0
   . data_src: 0x5080021
   ... thread: stack_test2.g.O:23828
   ...... dso: /root/abudanko/stacks/stack_test2.g.O3

I.e. the --user-regs=IP,SP,BP was being ignored, being overridden by the
needs of --call-graph=dwarf.

After applying the change in this patch the sample data contains the
user specified register, but making sure that at least the minimal set
of register needed for DWARF unwinding (DWARF_MINIMAL_REGS) is
requested.

The user is warned that DWARF unwinding may not work if extra registers
end up being needed.

  -g call-graph dwarf,K                         full_regs
  --user-regs=user_regs                         user_regs
  -g call-graph dwarf,K --user-regs=user_regs   user_regs + DWARF_MINIMAL_REGS
<REST remains the same>

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